Any space, enclosed or open, can be vulnerable to the dispersal of harmful airborne biological agents. Silent and near-invisible, these bioagents can sicken or kill living things before steps can be taken to mitigate their effects. Venues where crowds congregate are prime targets for biowarfare strikes engineered by terrorists but expanses of fields or forests could be victimized by an aerial bioattack.
Researchers have developed the Rapid Agent Aerosol Detector (RAAD), a highly sensitive and reliable trigger for the
U.S. military's early warning system for biological warfare agents. The trigger is the key mechanism because its continual monitoring of the ambient air in a location picks up the presence of aerosolized particles that may be threat agents. The trigger cues the detection system to collect particle specimens and then initiate the process to identify particles as potentially dangerous bioagents.
The RAAD determines the presence of biological warfare agents through a multistep process. First, aerosols are pulled into the detector by the combined agency of an aerosol cyclone that uses high-speed rotation to cull out the small particles, and an aerodynamic lens that focuses the particles into a condensed (i.e., enriched) volume, or beam, of aerosol. Then, a near-infrared (NIR) laser diode creates a structured trigger beam that detects the presence, size, and trajectory of an individual aerosol particle. If the particle is large enough to adversely affect the respiratory tract — roughly 1 to 10 micrometers — a 266-nanometer ultraviolet (UV) laser is activated to illuminate the particle and multiband laser-induced fluorescence is collected.
The detection process continues as an embedded logic decision, referred to as the “spectral trigger,” uses scattering from the NIR light and UV fluorescence data to predict if the particle's composition appears to correspond to that of a threat-like bioagent. If the particle seems threat-like, then spark-induced breakdown spectroscopy is enabled to vaporize the particle and collect atomic emission to characterize the particle's elemental content.
Spark-induced breakdown spectroscopy is the last measurement stage. This spectroscopy system measures the elemental content of the particle and its measurements involve creating a high-temperature plasma, vaporizing the aerosol particle, and measuring the atomic emission from the thermally excited states of the aerosol. The measurement stages are integrated into a tiered system that provides seven measurements on each particle of interest. Of the hundreds of particles entering the measurement process each second, a small subset of particles is down-selected for measurement in all three stages. The RAAD algorithm searches the data stream for changes in the particle set's temporal and spectral characteristics. If a sufficient number of threat-like particles are found, the RAAD issues an alarm that a biological aerosol threat is present.
To improve detection reliability, the RAAD team chose to use carbon-filtered, HEPA-filtered, and dehumidified sheathing air and purge air (compressed air that pushes out extraneous gases) around the optical components. This approach ensures that contaminants from the outside air do not deposit onto the optical surfaces of the RAAD, potentially causing reductions in sensitivity or false alarms.